1. Field of the Invention
This invention relates generally to the field of acoustic tumor detection. More particularly, the invention relates to a diagnostic technique for detecting small (.ltoreq.1 cm) intracanalicular tumors.
2. Prior Art
Until recently, the auditory brainstem response (ABR) test was an important component of the clinical test battery for acoustic neuromas. Early studies reported detection in the 95%-98% range, but these tumors were typically fairly large. In "Electrocochleography and Auditory Brainstem Electric Responses in Patients with Pontine Angle Tumors", Ann Otol Rhinol Laryngol 1980;89 (suppl 75):1-19, J. J. Eggermont, M. Don, and D. E. Brackmann examined the impact of tumor size on detection using estimates from CT scans and surgical reports and concluded that tumors smaller than 1.0 cm often go undetected by standard clinical ABR methodology. This conclusion has been supported by more recent and extensive studies that compared the sensitivity of this ABR methodology with gadolinium (Gd-DTPA) enhanced magnetic resonance imaging (MRI). Using standard measures of peak and inter-peak latencies, waveform morphology, or presence of waves, the ABR tests were nearly 100% accurate in detection of all extra- and intracanalicular tumors larger than 1.0 cm. However, for intracanalicular tumors smaller than 1.0 cm, the accuracy of some of these standard latency measures varied across studies from 63% to 93% with corresponding false negative rates ranging from 7% up to 37%. This wide range of detection and false positive rates is probably due to the different criteria selected. The high failure rate in detecting small intracanalicular tumors in some of these studies is not surprising since normal ABR latencies are possible if they are determined by a frequency region of the cochlea not affected by the tumor. Possible mechanisms underlying latency changes caused by tumors have been discussed and reviewed in "Efficacy of Auditory Brainstem Response as a Screening Test for Small Acoustic Neuromas", Am J Otol 1995; 16:136-139 by M. L. Gordon and N. L. Cohen.